The above example starts with the last-referenced global (in this case, process-private
globals): ^||a(1,1). $ZORDER does not contain the value of ^||a(1,1),
but works forward from that point. Calls to $ZORDER traverse
the subscript tree nodes in the following order: (1,2), (2), (2,1), (2,1,1), (2,1,2),
(2,2), (3), (3,1). Each WRITE $ZORDER displays the data value in
each successive node. It then runs out of nodes and generates the following error: <UNDEFINED>
^||a(3,1). Note that ^||a(3,1) is not undefined; it
is specified because $ZORDER could not find another global after
this one.